2. 自考
  3. The basic principle of radar is illustrated by what happens when one shouts in a cave. The echo of the shouting (11)______ the

The basic principle of radar is illustrated by what happens when one shouts in a cave. The echo of the shouting (11)______ the

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问题      The basic principle of radar is illustrated by what happens when one shouts in a cave. The echo of the shouting  (11)______  the walls helps a person determine the size of the cave. With radar，however，the waves are radio waves instead of  (12)______  waves. Radio waves travel  (13)______  the speed of light，about300,000 kilometers in a second. A radar set  (14)______  a short burst of radio waves. Then it receives the echoes  (15)______  when the waves bounce(反射) off objects. By determining the time it  (16)______  for the echoes to return to the radar set，a trained technician  (17)______  determine the distance between the radar set and other objects. The word“radar”，  (18)______  ，gets its name from the term“radio detection and ranging. ”“Ranging”is the term for detection of the  (19)______  between an object and the radar set.  
     Besides being of critical importance to pilot，radar is  (20)______  for air traffic control，for tracking ships at sea，and for tracking weather systems and storms.   
选项 A、in case
B、in turn
C、in short
D、in fact
答案D
解析 in fact：事实上，实际上；in short：总而言之，in rum：轮流；in case：假使，以防万一。
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